1. Field of the Invention
The present invention relates generally to heating devices, and in particular to a band heater with a strap assembly longitudinally slidably mounted on a shoe assembly for accommodating thermal expansion and contraction.
2. Description of the Prior Art
Various types of thermal control devices have been produced to accommodate a wide variety of heating and cooling system applications. Band heaters comprise a class of thermal control devices. They are characterized by their annular configurations designed for encircling objects to be heated and cooled. Typical band heater applications include heating the barrels of extrusion and injection molding machinery, heating blown film dies, and heating other cylindrical objects. Prior art band heaters have also been employed for cooling objects on which they are mounted. For applications which require both heating and cooling, prior art band heaters have been provided with both coolant-conveying cooling tubes and electrical heating elements.
A problem commonly encountered in designing band heaters relates to securely clamping them on the objects being heated and cooled. A common band heater design includes an annular shoe assembly comprising multiple, discrete shoe segments. The shoe assembly is retained in place encircling the barrel of an object to be heated or cooled by an annular strap assembly. Prior art strap assemblies include screw-type mechanical fasteners for exerting inwardly-directed radial forces whereby the shoe assemblies could be securely clamped in heat-exchange relationships with the cylindrical object barrels.
A common problem with previous band heaters relates to slippage between the shoe assemblies and the strap assemblies thereof. Such relative slippage can be caused by thermal expansion and contraction as the band heater components heat up and cool down. The practice of using different metals and alloys for the shoe and clamp assemblies tends to contribute to the relative slippage problem. For example, a typical design includes aluminum shoe segments and a stainless steel strap assembly. However, aluminum, with its higher coefficient of thermal expansion, expands up to five times faster than steel. Thus there is a need for a band heater design which accommodates such differential thermal expansion and contraction.
A prior art solution for mounting strap assemblies on shoe assemblies involves tapping the shoe segments to receive screws for mounting the strap assembly. However, this design often presented assembly problems, particularly with relatively large band heaters of up to 72 inches in diameter. Moreover, screw-mounted strap assemblies were not well-adapted to accommodate differential thermal expansion and contraction.
Heretofore there has not been available a band heater with the advantages and features of the present invention. The present invention addresses the aforementioned problems with prior art band heaters.